buying a prepaid SIM card for UNLOCKED phone.

[disneyred]

hello.
I would like to buy a prepaid SIM card for my unlocked GSM phone...
(i am assuming its easy just to slip this card into my phone and start using it !)

does anyone know where in and around WDW I can get this ??
I assume the prices for these are similar and there is no need to shop around ?

any ideas of what is the best operator around WDW is ??

thanks !